The present invention relates to devices for illuminating dot-shaped surface elements on a moving photosensitive record carrier.
It is already known to provide an optical printer with light switching elements each composed of a plurality of light switching cells. These light switching cells receive light from a light source via optical fibers.
The light switching cells can be selectively excited by control means including a character generator so that the light beams made available by the optical fibers are transferred by means of an imaging lens system to a movable photosensitive record carrier. e.g. an electrophotographic intermediate carrier.
In such printers, particularly at least if the light switching elements are arranged in a row, the imaging lens systems have an effective diameter which corresponds approximately to the length of the associated light switching cell. In order to overcome the thus occurring illuminating problems of the individual light switching arrays, it has already been proposed, as shown in FIG. 1, to provide photocouplers 7 between the light switching elements 1 and the photoconductor strips 5 which are comprised of a plurality of optical fibers 3 so as to better guide the light to the spatially very small light switching cells 4. The photocouplers 7 have a rectangular shape and project beyond the ends of the light switching cells so that the arrangement becomes less tolerance sensitive. The light switching cells 4 of the light switching elements 1 modulate the resulting light beams 19 which are transmitted, via an imaging lens system 11, to a photosensitive record carrier 13.
As shown in FIG. 1, each light switching cell 4 receives light from several light conductive fibers 3 since the latter emit light beams which propagate in the form of cones. The result is that the light switching cells 4 disposed in the edge regions of a light switching element 1 can be illuminated only from one side with half a light cone. Moreover, not all light beams impinging on the lateral surfaces 15 and 17 of the photocouplers 7 are totally reflected. Therefore the imaging lens system 11 also receives less light in the edge regions.
Then there are other problems that may result from light losses during reflection, from the spacing of the light exit surface 9 of a photocoupler 7 from the light switching cells 4 of a light switching element 1, and from the dissecting effect that may occur at the reflection surfaces 15 and 17.
Since the light switching cells 4 do not provide enough light for the imaging lens system in the edge regions, a charged photosensitive record carrier 13 is illuminated unevenly. The result is that because of the break in light intensity in the transition regions shown in FIG. 3, i.e. the regions between the lens systems 11 of FIG. 1, not enough discharges take place in the production of the charge image on the intermediate carrier 13 so that darker stripes may be produced on the record carrier during toner development, as shown in FIG. 2.